how is iMessage fundamentally any different from texting
Not entirely sure what you're asking but
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iOS does not allow you to use any other messaging app for SMS. This is surely intentional to lock you into iMessage.
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If you're messaging iOS --> iOS your "text" messages (SMS) are automatically upgraded to the iMessage protocol, and there are a wide variety of features that are enabled without the user downloading any other apps or switching the protocol. It just happens.